Dieting is the practice of eating (and drinking) in a regulated fashion to achieve a particular, short term objective. This is distinct from the more basic concept of "diet," which addresses the longer term and more generic habit of nutritional consumption. For example, vegan eats a diet completely devoid of animal products, including milk; but while this is a diet, it is not "dieting."

The most common objective of dieting is loss of excess body fat. There are also special religious diets - observant Jews, for example, must not eat certain foods during Passover when special dietary rules are in effect; and other religious-based dietary restrictions apply throughout the year to Jews, Muslims, and Hindus, among others. Some diets are prescribed for particular medical reasons, such as sodium-free diets, bland diets and soft food diets. Some dieting is actually designed to promote increase of body fat and/or of muscular weight gain.

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Weight-loss diets restrict the intake of specific foods, or food in general, to reduce body weight. This is what "fad diets" are marketed for. There is a (sometimes confusing) multitude of weight loss techniques, many of which are ineffective. What works for one person will not necessarily work for another, due to metabolic differences and lifestyle factors. Also, it's important to note that dieting does not actually lead to weight loss in the long term. Reducing the body's food supply causes it to stockpile excess fat as a starvation response.

Certain religions (such as Judaism, Hinduism and Islam) impose strict restrictions on food choices and preparation, in accordance with edict and tradition. For instance, in Judaism's discipline, you should not mix meat and any kinds of food that is made by cheese together when having a meal. Pork, shellfish, and food which is made with animal's blood are also forbidded. In Hinduism, people should not eat beef because they think cattle is sacred and inviolable. In Islam, it's quite similar to Judaism that people can not eat pork, and alcohol is also not allowed.

Vegetarianism is typically adopted in pursuit of general good health, for spiritual or ethical reasons, when other food choices are not available, or as a matter of personal taste.

Anorexia and bulimia are psychological (and possibly neurological) disorders in which victims endanger their lives through dangerous diet patterns.

Many professional athletes impose weight-gain diets on themselves. For example, wrestlers may overeat in order to achieve a higher weight class. American Football players may try to "bulk up" through weight-gain diets in order to gain an advantage on the field with a higher mass.

Medical conditions often require the following of special diets. Each of these such diets will specifically include or exclude or regulate certain chemicals (and the foods that contain them). For example, a person who has diabetes is often on a diet designed to carefully manage their blood sugar level. Epileptics are often put on the Ketogenic Diet. Sufferers of celiac disease must follow a gluten-free diet, the lactose-intolerant are advised to omit milk products, and people with kidney disease must follow a strict low-sodium diet to ease the strain on their kidneys. Treatment of mild hypertension includes adhering to a diet rich in fruits and vegetables and low in fat and sodium. This diet may be tailored to focus on weight loss if that is necessary to control blood pressure.

The practice of dieting in order to lose weight is ancient in its origins. Throughout the 17th and 18th centuries, physicians and patients regulated their food carefully, in order to prevent disease. In the 19th century, as the scientific classification of foods took shape, doctors and scientists began experimenting with targeted diets.

A successful weight-loss diet requires that energy expenditure exceeds energy intake (from food). In theory, one must burn 3,500 Calories (14,644 kilojoules) more than one consumes to lose one pound (0.45 kilograms) or burn 9000 Calories (37,656 kilojoules) more than one consumes to lose one kilogram. [dubious — see talk page]

Actual numbers are a bit less, since fat tissue is not 100% fat, so burning 9000 Calories will make one lose a bit more than 1Kg. But 9000Kcal per Kg is a good rule of thumb.

According to the principles of thermoregulation, humans are endotherms. We expend energy to maintain our blood temperature at body temperature, which is about 37 °C (98.6 °F). This is accomplished by metabolism and blood circulation, by shivering to stay warm, and by sweating to stay cool.

In addition to thermoregulation, humans expend energy keeping the vital organs (especially the lungs, heart and brain) functioning. Except when sleeping, our skeletal muscles are working, typically to maintain upright posture. The average work done just to stay alive is the basal metabolic rate, which (for humans) is about 1 watt per kilogram of body mass. Thus, an average man of 75 kilograms who just rests (or only walks a few steps) burns about 75 watts (continuously), or about 6,500 kilojoules (1,500 Calories) per day.

The ability of a few hours a week of exercise to contribute to weight loss can be overestimated. To illustrate, consider a 100-kilogram man who wants to lose 10 kilograms and assume that he eats just enough to maintain his weight (at rest), so that weight loss can only come from exercise. Those 10 kilograms converted to work are equivalent to about 350 megajoules. (We use an approximation of the standard 37 kilojoules or 9 Calories per gram of fat.) Now assume that his chosen exercise is stairclimbing and that he is 20 percent efficient at converting chemical energy into mechanical work (this is within measured ranges). To lose the weight, he must ascend 70 kilometers. A man of normal fitness (like him) will be tired after 500 meters of climbing (about 150 flights of stairs), so he needs to exercise every day for 140 days (to reach his target).

The minimum safe dietary energy intake (without medical supervision) is 75 percent of that needed to maintain basal metabolism. For our hypothetical 100-kilogram man, that minimum is about 5,700 kilojoules (1,300 Calories) per day. By combining daily aerobic exercise with a weight-loss diet, he would be able to lose 10 kilograms in half the time (70 days). Of course, the described regime is more rigorous than would be desirable or advisable for many persons. Therefore, under an effective but more manageable weight-loss program, losing 10 kilograms (about 20 pounds) may take as long as 6 months.

There are also some easy ways for people to take exercise, such as walking rather than driving, stairclimbing istead of taking elevators, doing more housework, or parking the cars farer and walking to school or office can all be very good exercise.

It is important to understand the difference between weight loss and fat loss. Weight loss typically involves the loss of fat, water and muscle. A dieter can lose weight without losing much fat. Ideally, overweight people should seek to lose fat and preserve muscle, since muscle burns more calories than fat. Generally, the more muscle mass one has, the higher one's metabolism is, resulting in more calories being burned, even at rest. Since muscles are more dense than fat, muscle loss results in little loss of physical bulk compared with fat loss. To determine whether weight loss is due to fat, various methods of measuring body fat percentage have been developed.

Those on low-carbohydrate diets, and those doing particularly strenuous exercise, may wish to increase their protein intake. However, there may be risks involved. According to the American Heart Association, excessive protein intake may cause liver and kidney problems and may be a risk factor for heart disease[1]. There is no conclusive evidence that moderately high protein diets in healthy individuals are dangerous, however. It has only been shown that these diets are dangerous in individuals who already have kidney and liver problems. [2]

The energy humans get from food is limited by the efficiency of digestion and the efficiency of utilization. The efficiency of digestion is largely dependent on the type of food being eaten. Poorly chewed seeds are poorly digested. Refined sugars and fats are absorbed almost completely. Despite the claims of certain popular diets, chewing and digesting does not use a substantial amount of the energy offered by any food (that anyone would want to eat). Even celery, known for being low in caloric value, contains enough sugars (including sucrose, glucose and galactose) to easily compensate for the cost of (energy invested in) chewing it. (See Food & Nutrition Information Center.}

The efficiency of energy utilization by skeletal muscles is around 20 percent. That is, of the chemical energy used, 20 percent does work and 80 percent creates heat.

Any diet that fails to meet minimum nutritional requirements can threaten general health (and physical fitness in particular). If a person is not well enough to be active, weight loss and good quality-of-life will be unlikely.

Sometimes dieters will take excessive amounts of vitamin and mineral supplements. While this is usually harmless, some nutrients are notably dangerous. Men (and women who don't menstruate) need to be wary of iron poisoning. Retinol (oil-soluble vitamin A) is toxic in large doses.

A sensible weight-loss diet is a normal balanced diet; it just comes with smaller portions. Extreme diets (based on caprice or obsession) will likely lead to malnutrition.

All body processes require energy to run properly. When the body is expending more energy than it is taking in (e.g. when exercising), body cells rely on internally stored energy sources, like complex carbohydrates and fats, for energy. The first source the body turns to is glycogen, which is a complex carbohydrate created by the body. When that source is nearly depleted, the body begins lipolysis, the metabolism of fat for energy. In this process, fats, obtained from fat cells, are broken down into glycerol and fatty acids, which can be used to make energy. The primary by-products of metabolism are carbon dioxide and water; carbon dioxide is expelled through the respiratory system.

Diets affect the "energy in" component of the energy balance by limiting or altering the distribution of foods. Techniques that affect the appetite can limit energy intake by affecting the desire to overeat.

Consumption of low-energy, fiber-rich foods, such as non-starchy vegetables, is effective in obtaining satiation (the feeling of "fullness"). Exercise is also useful in controlling appetite. (Extreme physical fatigue, such as experienced by soldiers and mountain climbers, can make eating a difficult chore.)

The use of drugs to control appetite is (potentially) dangerous. Stimulants are often taken as a means to ignore (normal, healthy) hunger by people who are not actually overweight.

Habitual or emotional eating is a common problem. Sufferers often turn to self-help books, hypnosis and group therapy. While these sources can sometimes be of assistance, dieters must beware. Some "diet gurus" are charlatans, others are well-meaning but focus on psychology or philosophy at the expense of practical solutions. Diets designed to appeal to people emotionally are often either very difficult to follow (i.e., too strict) or useless (i.e., too lenient).

There exist both profit-oriented and non-profit weight loss organizations who assist people in their weight loss efforts. Examples of the former include Weight Watchers and Jenny Craig; examples of the latter include Overeaters Anonymous, as well as a multitude of non-branded support groups run by local churches, hospitals, and like-minded individuals.

These organizations' customs and practices differ widely. Some groups are modelled on twelve-step programs, while others
are quite informal. Some groups advocate certain prepared foods or special menus, while others train dieters to make healthy choices from restaurant menus and while grocery-shopping and cooking.

Most groups leverage the power of group meetings to provide counseling, emotional support, problem-solving, and useful information.

Popular diets (sometimes pejoratively called "fad diets") usually derive their popularity from the personalities of their proponents. These proponents include "diet gurus" and celebrity converts. "Diet books" are the primary means of communicating the specifics of popular diets.

Most popular diets experience short-lived popularity, partly because new diet books are continuously being published.

Judging the effectiveness (and nutritional merit) of popular diets can be especially difficult. Diet proponents often locate medical professionals to back up their work. And some diets are so controversial that they divide the medical community.

Many popular diets advocate the combination a specific technique (such as eliminating a certain food, or eating only certain combinations of foods) with reduced caloric intake, with the goal being to accelerate weight loss. Others ignore traditional science altogether.

Low-fat diets were popular during the 1980s and 1990s, encouraging people to eat foods low in fat (or without fat altogether) and instead eat foods high in carbohydrates.
For isntance, people should eat less fat junk food or sweet snacks, instead, you can choose low-calorie, and high-fiber foods like fruits and vegetables. These will help people feel full longer, and make any diet plan more effective. Also, plan your meals and buy the food you need so that you will not be tempted to turn to fast food when you are hungry.

The general public came to believe, partly due to information from low-fat diet proponents, that carbohydrates were "energy food" and that only fat made people fat. This led to high consumption of low-fat foods rich in refined carbohydrates (notably corn syrup), which led some people to gain more weight.

Some low-fat diets were healthier, focusing on consumption of whole grains, vegetables and lean meats. (See Pritikin diet.) But even these diets did not recognize the importance of essential fatty acids.

For example, the uncooked diet tends to improve a participant's health. People's weight tends to nomalize due to the lack of fat in the diet. It also gives people a high amount of energy as uncooked food is easily matabolized.

The Atkins Diet is a very popular diet.
Dr. Robert Atkins' concept, somewhat exaggerated by the media, that a person can lose weight whilst gorging on meat, has captured the public's imagination. The success of those who tried the diet varied depending on the degree they adhered to the long term stages of the diet structure. The Atkins diet was originally designed for diabetes patients who wanted to manage their insulin levels more effectively. The diet was also embraced by those seeking a diet that allows eating to satiation.

Atkins discourages refined carbohydrate intake and encourages protein intake, especially in the form of meat. The diet encourages the consumption of fruits and non-starchy vegetables for the provision of fiber and nutrients; it takes a somewhat neutral stand on fat intake.

Many people experience rapid initial weight loss on Atkins, some of which is due to depletion of glycogen stores in the liver. Loss of glycogen is associated with loss of water weight, since the body stores up to four pounds of water for each pound of glycogen.

Low carbohydrate diets have been shown to reduce the fasting levels of triglycerides. Elevated triglycerides are a demonstrated risk factor for heart disease. (Low-fat diets also reduce fasting levels of triglycerides.)

A low-carbohydrate diet may not be suitable as a weight-maintenance diet (long-term). The products of fat metabolism (lipolysis) and protein metabolism (gluconeogenesis) include ketones which can be harmful.

Note: Any successful weight-loss diet will cause some acidosis; symptoms range from mild fatigue to severe joint pain. Acidosis can be controlled by drinking water (in large amounts) and taking antacid supplements (or eating vegetables grown in alkaline soil).

Note: Human metabolism is enormously complicated. Diets whose effectiveness is not based on the simple balance of energy must be evaluated experimentally. The premise that protein is less fattening than carbohydrates is unproven, although the specific dynamic action of protein is 30%, while the corresponding figures for carbohydrate and fat are 6% and 4% respectively. ""One hundred kilocalories of protein produces an extra 30 kcal of heat, while similar amounts of carbohydrate and fat raise the metabolic rate by 6 and 4 kcal, respectively" "Essential Human Anatomoy and Physiology" Barbara R. Landau, 1976

Since the advent of controversial diets such as Atkins, various diets that stress the eating habits of "natural humans" have been developed. The Evolution Diet explains "what and how we were designed to eat"; the Paleolithic Diet imitates the way people ate during the Stone Ages. These eating plans include basically natural foods (those not processed by humans). Whereas the Paleolithic Diet exludes milk and grain-foods, The Evolution Diet excludes human-made ingredients such as partially hydrogenated oils but allows some processed foods such as whole-grain crackers and dairy products. Anthropologists who focus their research on human evolution, however, are quick to point out that the diet of Paleolithic peoples was most likely opportunistic. That is, these early humans would most likely eat whatever edible foods were available at any given moment (e.g. vegetables, termites, meat) and not restrict their intake of any food. Until recent human history, starvation has been a far greater threat than over-consumption.

There is a growing body of evidence that vegetarian diets can prevent obesity and lower disease risks.

According to the American Dietetic Association, "Vegetarians have been reported to have lower body mass indices than nonvegetarians, as well as lower rates of death from ischemic heart disease; vegetarians also show lower blood cholesterol levels; lower blood pressure; and lower rates of hypertension, type 2 diabetes, and prostate and colon cancer."

Vegans on average weigh 10 percent less than non-vegetarians. And in a year-long study comparing Dean Ornish's vegetarian diet to Weight Watchers, The Zone Diet, and The Atkins Diet, subjects on Dean Ornish's diet achieved the most weight loss (on average).

The Very Low Calorie Diet (VLCD) is a prescribed diet for obese patients. Daily intake consists of three milkshake-like formula drinks (made with powder concentrate and water), which supply about 2000 kilojoules (500 Calories) and all necessary vitamins and minerals.

There are risks to this diet. A patient who drinks more formula than allowed can get too much iron and selenium. Constipation is a problem: extra water and (fiber) laxatives may be required. Immune response may be compromised.

VLCD should only be used for dieting when a patient's body mass index exceeds 30. The diet requires regular consultation between patient and doctor.

VLCD can be very successful when used over a 6-12 week period. As with all starvation diets, metabolism will fall. A sensible diet-and-exercise plan must follow cessation of VLCD, or weight will be gained back.

There are many diet pills for sale, some which are associated with comprehensive dietary programs. Many such pills, including many of those containing vitamins and minerals, are not effective for losing weight.

Some drugs enable short-term weight loss, usually with unpleasant and potentially dangerous side effects. The drugs include (physiologically active) herbal products available at health food stores, as well as over-the-counter (OTC) and prescribed medications provided by doctors and pharmacists.

Typically these drugs fall into two classes: diuretics to induce water-weight loss and stimulants (such as ephedrine, and more recently synephrine, due to the former's ban as a weight loss supplement by the FDA; although ephedrine is still available as an asthma medication) to increase heart rate and reduce appetite. Both classes of drugs can cause kidney and liver damage, and stimulants can cause sudden heart attacks, addiction, and both ephedrine and synephrine have been proven to cause ischemic stroke.

Yo-yo dieting is defined by alternating periods of feast and famine (that the dieter deliberately undertakes). It is a particularly ineffective method of sustaining weight loss.

The human body responds to starvation by decreasing metabolism. When food is again available, it is stored immediately as fat. This survival mechanism, while a useful response to genuine food scarcity, leaves the yo-yo dieter feeling lethargic and fatigued (and defeated).

Metabolism can be restored to a higher level with exercise and a sensible weight-loss diet. This diet is defined by the minimum safe daily caloric intake of 75 percent of the basal metabolic rate or 4200 kilojoules (1000 Calories), whichever is greater. (Those eating less should do so only under medical supervision. Parents and guardians should consult medical professionals before placing their children on any type of diet.)

Once an ideal weight is attained, a weight-maintenance diet is essential. This requires limiting excess caloric intake and making small changes in caloric intake in response to physical observations (of one's weight and appearance).

Scientific analysis of the dangers of fasting (and discussion of partial fasting with protein supplementation)Edit

While anyone can lose weight by fasting (temporarily stopping one's food intake), it is a dangerous practice. When concentration camp survivors, who involuntarily suffered famine as a result of horrendous living conditions, were examined by doctors, what little weight they had was mostly fat, with practically no muscle.

The muscle loss is partly due to the fact that the brain cannot rely completely on fat for fuel. The brain usually reserves ketones for lipid synthesis but will use ketones (from fat) for some energy once levels rise during carbohydrate shortages or starvation, but it must get at least 15 percent of its energy from glucose, and it takes a much greater percentage than this early in a fast before the switch to ketones for most energy needs. Glucose can only be synthesized from proteins, glycerol and carbohydrates.

The body stores carbohydrates as fat and as glycogen in the muscles and the liver; glycogen is used to make glucose. Glycogen stores (from carbohydrates) can only last a couple days (during starvation). (In fact, marathon runners experience a shortage of easily-available glycogen after only 2 hours, commonly called "hitting the wall" or bonking.)

Because fasts, very low calorie diets (VLCD), and low-carbohydrate diets restrict the intake of carbohydrates, glucose must be obtained from protein. If dietary protein is insufficient, internal sources will be obtained: autolysis and muscle wasting occurs. (The conversion of amino acids to glucose is called gluconeogenesis.)

A very low calorie diet that restricts all carbohydrates and non-essential fats, while providing just enough dietary protein to prevent muscle loss, is termed a "protein sparing modified fast" (PSMF).

After experimentation, it was found that a protein intake of 1 to 1.5 grams of protein per kilogram of ideal bodyweight (lean body mass or LBM) prevented the loss of body protein. A somewhat "safer" intake of .8 to 1.2 grams of protein per pound of LBM is often recommended. (More active persons and those taking less protein must compensate by consuming at least some carbohydrates.)

Thus, PSMF allows for rapid fat loss due to the severe caloric deficit that is created when nearly all carbohydrates and fats are removed from the diet. This extreme dieting technique has many potential hazards, such as hormonal changes and rapid metabolic slowdown. PSMF is sometimes used by bodybuilders to "get cut" (lose fat to expose muscle) just before competitions.